The present invention relates to a niobium sintered body that can provide a preferable leakage current value in spite of the large capacitance, a production method therefor, and a capacitor using the sintered body.
Capacitors for use in electronic apparatus such as portable telephones and personal computers are required to be small in size and large in capacitance. Of those capacitors, a tantalum capacitor is preferably used, because the capacitance is large, not in proportion to the size, and the tantalum capacitor also has good characteristics. The tantalum capacitor usually employs a sintered body of tantalum powder as an anode. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body.
The increase in mass of the sintered body inevitably enlarges the shape of the capacitor, so that the requirement for a small-sized capacitor is not satisfied. One approach to solve these problems is a capacitor using a material which has a greater dielectric constant than tantalum. One material which has such a greater dielectric constant is niobium.
Japanese Laid-Open Patent Application No. 55-157226 discloses a method for producing a sintered element for a capacitor. This method comprises the steps of subjecting a niobium powder ranging from an agglomerate to fine particles with a particle diameter of 2.0 xcexcm or less to pressure molding and sintering, finely pulverizing the molded sintered body, connecting a lead to the finely pulverized particles of the sintered body, and thereafter sintering the connected body again. However, the above-mentioned application does not describe detailed characteristics of the obtained capacitor.
U.S. Pat. No. 4,084,965 discloses a capacitor using a niobium powder with a particle diameter of 5.1 xcexcm obtained from a niobium ingot through hydrogenation and pulverizing. However, the niobium sintered body has a high LC value, so that the serviceability of the niobium sintered body is regarded as poor.
The inventors of the present invention have already proposed to improve the leakage current characteristics (hereinafter referred to as an LC value) of niobium by partially nitriding the niobium and the other like manners (Japanese Laid-Open Patent Application No. 10-242004, U.S. Pat. No. 6,115,235). The LC value can be further decreased, for example, by increasing the sintering temperature in the preparation of the above-mentioned niobium sintered body. However, with the increase of sintering temperature, a product of a capacitance per unit mass of the obtained sintered body and a forming voltage to form a dielectric on the surface of the sintered body (hereinafter abbreviated as a CV value) becomes smaller. As a result, it is difficult to achieve the final goal, that is, to obtain a well-balanced niobium sintered body having a high CV value and a low LC value. When a capacitor is made from a niobium sintered body which has been prepared only with an aim to obtain a high CV value, there is a problem that a capacitor having an exceptionally large LC value will be produced.
Therefore, an object of the present invention is providing a niobium sintered body with a preferable leakage current value (LC value) in spite of the large capacitance, a production method therefor, and a capacitor using the above-mentioned sintered body.
The inventors of the present invention have intensively studied the above-mentioned problems, and found an unprecedented sintering method suitable for niobium powder used for capacitors where the niobium powder is allowed to stand at a maximum sintering temperature for predetermined time, and then accomplished the present invention.
Namely, the present invention provides the following niobium sintered body, a production method therefore, and a capacitor using the sintered body.
[1] A niobium sintered body prepared by sintering a niobium powder, wherein a product (CV) of a capacitance (C: xcexcF/g) per unit mass and a forming voltage (V: volt(V)) is 90,000 xcexcFxc2x7V/g or more, and a value obtained by dividing a product of a mean particle diameter (D50: m) of a primary particle of the niobium powder and a leakage current (LC: xcexcA/g) by the CV value is 5xc3x9710xe2x88x924 xcexcmxc2x7A/(xcexcFxc2x7V) or less.
[2] The niobium sintered body as described in the above-mentioned aspect [1], wherein the niobium powder is partially nitrided.
[3] The niobium sintered body as described in the above-mentioned aspect [2], wherein a nitrogen content in the niobium powder is within a range of 20 mass ppm to 200,000 mass ppm.
[4] The niobium sintered body as described in the above-mentioned aspect [3], wherein a nitrogen content in the niobium powder is within a range of 500 mass ppm to 7,000 mass ppm.
[5] A method for producing a niobium sintered body comprising the step of sintering a niobium powder at high temperature, wherein the niobium powder is sintered at a temperature of 500xc2x0 C. to 2,000xc2x0 C. and allowed to stand at a maximum sintering temperature for 60 minutes to 150 minutes.
[6] A method for producing a niobium sintered body as described in the above-mentioned aspect [5], wherein the niobium powder is sintered at a temperature of 900xc2x0 C. to 1500xc2x0 C. and allowed to stand at a maximum sintering temperature for 80 minutes to 130 minutes.
[7] The method for producing the niobium sintered body as described in the above-mentioned aspect [5] or [6], wherein the niobium powder is granulated to have a primary particle with a mean particle diameter of 3 xcexcm or less.
[8] The method for producing the niobium sintered body as described in the above-mentioned aspect [7], wherein the niobium powder is granulated to have a primary particle with a mean particle diameter of 3 xcexcm to 0.1 xcexcm.
[9] The method for producing the niobium sintered body as described in any one of the above-mentioned aspect [5] to [8], wherein the niobium powder is partially nitrided.
[10] The method for producing the niobium sintered body as described in the above-mentioned aspect [9], wherein a nitrogen content in the niobium powder is within a range of 20 mass ppm to 200,000 mass ppm.
[11] The method for producing the niobium sintered body as described in the above-mentioned aspect [10], wherein a nitrogen content in the niobium powder is within a range of 500 mass ppm to 7,000 mass ppm.
[12] A capacitor comprising an electrode comprising the niobium sintered body as described in any one of the above-mentioned aspect [1] to [4], a dielectric provided on the surface of the sintered body, and a counter electrode provided on said dielectric.
[13] The capacitor as described in the above-mentioned aspect [12], wherein the dielectric comprises niobium oxide formed by electrolytic oxidation.
[14] The capacitor as described in the above-mentioned aspect [12], wherein the counter electrode comprises at least one material selected from the group consisting of an electrolytic solution, an organic semiconductor, and an inorganic semiconductor.
[15] The capacitor as described in the above-mentioned aspect [14], wherein the counter electrode comprises the organic semiconductor, which is selected from the group consisting of an organic semiconductor comprising benzopyrroline tetramer and chloranil, an organic semiconductor comprising as the main component tetrathiotetracene, an organic semiconductor comprising as the main component tetracyanoquinodimethane, and an organic semiconductor comprising as the main component an electroconducting polymer prepared by doping a polymer having at least two repeat units represented by general formula (1) or (2) with a dopant: 
wherein R1 to R4 which may be the same or different each independently represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, or an alkoxyl group having 1 to 6 carbon atoms; X represents an oxygen atom, a sulfur atom, or a nitrogen atom; and R5, which is present only when X is a nitrogen atom, represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and R1 and R2, and R3 and R4 may be independently combined to form a ring.
[16] The capacitor as described in the above-mentioned aspect [15], wherein the organic semiconductor is at least one material selected from the group consisting of polypyrrole, polythiophene, and substituted derivatives thereof.